In recent years, in the construction machinery such as the hydraulic excavator of this kind, with the aims of energy saving (lowering the fuel efficiency) and reducing the amount of the exhaust gas (carbon dioxide, nitrogen oxide, particulate matter, and the like for example) discharged from the engine and having the environmental load, so-called hybrid type construction machinery has been proposed in which a motor generator is made a power source in addition to an engine.
Also, a prior art on the hybrid type construction machinery of this type has been disclosed in Patent Literature 1. According to this Patent Literature 1, a hydraulic pump is driven by an engine, a hydraulic cylinder is driven by the pressure oil discharged from this hydraulic pump, and a generator for regeneration is driven by the pressure oil discharged from this hydraulic cylinder. More specifically, the generator for regeneration is driven by the energy which the pressure oil discharged from the hydraulic cylinder has, the motor generator is driven by the electric power generated by drive of this generator for regeneration to assist the engine, to be utilized for drive of the hydraulic pump, and to supply the electric power generated by the generator for regeneration to the battery for charging to be reused.